Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receive antenna receives a linear combination of these transmitted data streams.
In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire a channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation techniques. It is desirable for multiple antenna communication systems to co-exist with legacy single antenna communications systems (typically referred to as Single Input Single Output (SISO) systems). Thus, a legacy (single antenna) communications system must be able to interpret the preambles that are transmitted by multiple antenna communication systems. Most legacy Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”).
Among other benefits, OFDM systems are said to be resistant to the multipath effect of a wireless channel. To obtain this advantage, there is a guard interval in the preamble at the start of each OFDM symbol. The guard interval, however, sacrifices the efficiency of the system. Thus, system efficiency can be increased by either reducing the guard interval, which compromises the resistance to the multipath effect, or by increasing the OFDM symbol duration.
In the current 802.11a/g standard, each channel is 20 MHz wide with 64 subcarriers, which leads to an OFDM symbol duration of 3.2 μs. One proposal to increase throughput is via channel bonding. In such a scheme, the bandwidth increases to 40 MHz and the number of subcarriers to 128. However, the symbol duration in 40 MHz is still 3.2 μs. Although the link throughput doubles in 40 MHz, the efficiency of the system does not increase since the guard interval duration and symbol duration do not change. If the same guard interval duration is maintained, the symbol time can be increased to increase the system efficiency. For example, by increasing the number of subcarriers to 256, the symbol time increases to 6.4 μs. If the Guard Inteval is held constant at 0.8 μs, the throughput (and the efficiency) increases by 11.11%. Hence, if the number of subcarriers used in 40 MHz is increased to 256, a need exists for a training mechanism that covers all 256 subcarriers.